I was just walking through a wild area in Seattle with lots of weeds, and came across some strange caterpillars. They are mostly hairless (to the naked eye, anyway), and are striped black over orange with black legs. Can you tell me what they are?

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I believe you may have seen the Cinnabar moth caterpillar, Tyria jacobaeae, which was introduced to the United States to control a noxious weed, tansy ragwort (Senecio jacobaea). Here are pictures to compare, and more information:WikipediaFlickr image

I have what I believe to be Cinnabar moth larvae eating my Virginia creeper. How do I get rid of them? Everything I've seen on the web is how beneficial they are in controlling a weed, but nothing on how to the kill the pest.

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It's interesting that there are often unintended consequences when we import insects to control noxious weeds. The cinnabar moth was brought in to control tansy ragwort, Senecio jacobaea, now renamed Jacobaea vulgaris. See the following link from Oregon's agricultural experiment station:
"'The cinnabar moth arrived when threats to native plants did not receive much public or scientific scrutiny,' McEvoy said. Now a three-year survey conducted across 25 sites in western Oregon determined that cinnabar moths have been munching on arrow leaf groundsel, a native wildflower found principally in the mountains and occasionally on the coast. Scientists had thought that the places where arrow leaf groundsel grew were too cold for the cinnabar moth. But incremental climate change may raise temperatures enough to allow the cinnabar moth to thrive at higher elevations. Although the state hasn't reclassified the cinnabar moth as a pest, the story serves as a cautionary footnote in the tansy ragwort success story."

I don't believe much thought has yet gone into methods for controlling the moths when they devour non-target plants. I would guess that you could attempt to look for eggs and remove them manually, or attempt to control the larvae of the cinnabar moth (Tyria jacobaeae). We can't recommend pesticides as we are librarians and not licensed pesticide handlers, but you might investigate whether something like Btk (Bacillus thuringiensis kurstaki) is registered for controlling moths in their larval stage. Ohio State University Extension has information about Btk's use on gypsy moth. This should give you an idea of how Btk works.

British Columbia's Island Crop Management shows images of the moth at various stages, including the eggs of the moth, so you will know what to look for.
The British web page BugLife has information on the life cycle of the cinnabar moth:
"Females can lay up to 300 eggs, usually in batches of 30 or 60 on the underside of ragwort leaves. When the caterpillars (larvae) hatch they feed on the around the area of the hatched eggs but as they get bigger and moult (instars) they mainly feed on the leaves and flowers of the plant, and can be seen out in the open during the day.
Caterpillars are feeding from July – early September and are initially pale yellow but soon develop bright yellow and black stripes to deter predators.
The caterpillars feed on poisonous ragwort leaves. The poison from the leaves is stored in the caterpillar's body (and even remains when they are an adult moth). Any birds or other predators that ignore the caterpillars' bright warning sign will be repulsed by how foul they taste.
Numerous caterpillars on one ragwort plant can reduce it to a bare stem very quickly. They are also known to be cannibalistic.
The caterpillars overwinter as pupa in a cocoon under the ground. The adult moths emerge around mid May and are on the wing up until early August, during which time males and females will mate and eggs are laid."

You might also try to encourage birds in your garden, as they will eat some of the moths at caterpillar stage.